1. For the centres, stir the chocolate and cream together over low heat until melted and really smooth. Pour into a bowl and chill until firm, then scoop out teaspoonfuls and mould them around each cherry. (If the cherries are really large, you may need to halve them.) As you finish each one, put it on a baking paper-lined tray. Once they are all done, return them to the fridge to firm up.
2. In a bowl, stir together the macaroons, icing sugar and cocoa. Pour in the melted butter and rum and fold in well, then add the coconut and mix thoroughly. Mould large spoonfuls of this crumb mixture around the chilled chocolate centres so they are completely enclosed and as round as possible. Then return them to the fridge.
3. Melt the chocolate, butter and water together in a saucepan over low heat, stirring frequently, until silky smooth. Leave to cool to a thick coating consistency.
4. Dunk the balls one at a time into the cooled chocolate coating. Turn them over gently with a fork so they are completely covered. Then scoop them out, letting them drain briefly over the pan. Wipe the base of the fork carefully across the lip of the pan so that any excess coating drips away. Arrange the rumballs on the baking paper, and when they are all done chill them again to set the coating.
5. Now you can leave them as they are or decorate them. You can do all the usual things you do with chocolate truffles, such as rolling them in cocoa or dotting the tops with chocolate sprinkles or nuts. But for something really Christmassy, press tiny bits of edible gold or silver leaf on top of them (you’ll have to moisten any cocoa coated balls carefully or the foil won’t stick.) Then store them in an airtight container in the refrigerator until you need them. They’ll keep well for several weeks. When you serve them, try cutting one or two open so you can see the layers inside.